1. Field of the Invention
The present invention relates to a liquid crystal display, which is used for a display section of an electronic apparatus.
2. Description of the Related Art
A liquid crystal display has been used as a display section of a notebook computer, a TV receiver, a monitor for a personal computer, and a projector or the like. In recent years, a large-screen liquid crystal display panel has been manufactured and the demand of the liquid crystal display has been rapidly increased as the display section of the TV receiver. Therefore, the liquid crystal display is required for a higher visual quality. However, the liquid crystal display of a TN (Twisted Nematic) system, which was predominant display in the past, has a problem that it is difficult to obtain a display characteristic as the display section of the TV receiver because its viewing angle characteristics is not satisfactory. Therefore, in recent years, in order to obtain wide viewing angle characteristics, the technologies other than the TN system have been used for the liquid crystal display. One of these technologies is a technology referred to as a MVA (Multi-domain Vertical Alignment) system. In this MVA system, vertically aligning a liquid crystal molecular on a substrate when applying no voltage, if a voltage is applied to the liquid crystal, the alignment of the liquid crystal molecular is defined by a protrusion formed on the substrate or a slit provided on a transparent electrode (ITO).
Generally, in a vertical alignment system of vertically aligning the liquid molecular on the substrate, it has been known that an optical characteristic measured from an oblique direction about a normal line of a display screen is different from the optical characteristic in the normal line. Particularly, a gradation luminance characteristic viewed from the oblique direction, which is parallel or perpendicular to a polarizing axis is largely deviated from a gradation luminance characteristic viewed from a square direction.
In order to solve this problem, a liquid crystal display having a pixel structure including a pixel electrode that is electrically connected to a source electrode of a thin film transistor (TFT) for a pixel and a pixel electrode that is divided from that pixel electrode and is insulated from the source electrode has been known. According to this liquid display, a capacitance is formed by a pixel electrode that is insulated form the source electrode, the source electrode, and an insulation film sandwiched between the both electrodes. The pixel electrode insulated from the source electrode is driven by this capacitance.
FIG. 7 shows a structure of one pixel of a liquid crystal display having a pixel structure including divided two pixel electrodes. As shown in FIG. 7, a plurality of gate bus lines 106 and a plurality of drain bus lines 108 intersecting with the gate bus line 106 via an insulation film (not illustrated) are formed on a glass substrate 103. In the vicinity of the intersection of the gate bus line 106 and the drain bus line 108, a TFT 110 formed for each pixel is arranged. A part of the gate bus line 106 serves as a gate electrode (G) of the TFT 110. On the gate bus line 106, an operational semiconductor layer of the TFT 110 and a channel protective film (both of which are not illustrated) are formed via the insulation film. On the channel protective film of the TFT 110 above the gate electrode (G), a drain electrode (D) along with an n-type impurity semiconductor layer (not shown) underlying the same and a source electrode (S) along with an n-type impurity semiconductor layer (not shown) underlying the same are formed, the electrodes facing each other with a predetermined gap left between them.
In addition, a storage capacitor bus line 114 is formed across pixel regions defined by the gate bus lines 106 and the drain bus line 108 so as to be extended in parallel with the gate bus line 106. A storage capacitor electrode (intermediate electrode) 116 is formed above the storage capacitor bus line 114 for each pixel via an insulation film. The storage capacitor electrode 116 is electrically connected to the source electrode (S) of the TFT 110 through a control electrode 111. A storage capacitor Cs is formed by the storage capacitor bus line 114, the storage capacitor electrode 116, and the insulation film sandwiched between them.
A pixel region defined by gate bus lines 106 and drain bus lines 108 is divided into a sub-pixel A and a sub-pixel B. For example, the sub-pixel A, which has a trapezoidal shape, is disposed on the left side of a central part of the pixel region, and the sub-pixel B is disposed in upper and lower parts of the pixel region and at the right end of the central part excluding the region of the sub-pixel A. For example, the disposition of the sub-pixels A and B in the pixel region is substantially line symmetric about the storage capacitor bus line 114. A pixel electrode 121 is formed at the sub-pixel A, and a pixel electrode 123, which is separated from the pixel electrode 121, is formed at the sub-pixel B. The pixel electrodes 121 and 123 are both constituted by a transparent conductive film such as an ITO. An inter-electrode slit 126 is formed between the pixel electrode 121 and the pixel electrode 123. The inter-electrode slit 126 is formed within the pixel region.
The pixel electrode 121 is electrically connected to the storage capacitor electrode 116 and a source electrode (S) of the TFT 110 through a contact hole 118, on which a protective film (not illustrated) is opened. The pixel electrode 123 has a region which overlaps the control electrode 111 via the protective film and the insulation film. In the same area, a capacitance (a control capacitor) Cc is formed by the control electrode 111, the pixel electrode 123, and the protective film sandwiched between the electrodes 111 and 123.
A common electrode (not illustrated) is formed on an opposite glass substrate (not shown) provided opposite to the glass substrate 103. A linear protrusion 112a serving as an alignment regulating structure for regulating the direction of alignment of a liquid crystal is formed, which protrudes from the opposite glass substrate and is formed in a V-shape above the pixel electrode 121 on the left side of the central part of the pixel region. The linear protrusion 112a is formed so as to be substantially line symmetric about the storage capacitor bus line 114. In addition, a linear protrusion 112b is formed at a position opposite to the control electrode 111 that extends obliquely in the drawing. Further, a linear protrusion 112c is formed so as to protrude from the opposite glass substrate in a position in which it is substantially line symmetric with the linear protrusion 112b about the storage capacitor bus line 114.
An arrangement interval w1 of the linear protrusion 112a at the sub-pixel A and arrangement intervals w2 of the linear protrusions 112b and 112c at the sub-pixel B are formed so as to be substantially the same lengths. In FIG. 7, the arrangement interval w1 the interval is between the edge of the inter-electrode slit 126 and the edge of the linear protrusion 112a that is arranged within the sub-pixel A in adjacent to the inter-electrode slit 126. In the same way, the arrangement interval w2 is the interval between the edge of the inter-electrode slit 126 and each of the edges of the linear protrusions 112b and 112c that are arranged within the sub-pixel B in adjacent to the inter-electrode slit 126. For example, the arrangement intervals w1 and w2 are formed at 25 μm.
At the sub-pixel A, a liquid crystal capacitance Clc1 is formed between the pixel electrode 121, the common electrode, and the liquid crystal sandwiched between the both electrodes. At the sub-pixel B, a liquid crystal capacitance Clc2 is formed between the pixel electrode 123, the common electrode, and the liquid crystal sandwiched between the both electrodes. The liquid crystal capacitance Clc2 is series-connected to the control capacitor Cc between the glass substrate 103 and the opposite glass substrate.
When the TFT 110 is turned on, the electric potentials of the source electrode (S) and the control electrode 111 are the same as that of a gradation voltage VD applied to the data bus line 108, and at the same time, the electric potential of the pixel electrode 121 electrically connected is also the same as that of the gradation voltage VD. To the liquid crystal capacitance Clc1, a voltage depending on the electric potential difference applied between the pixel electrode 121 and the common electrode is applied. For example, assuming that the voltage applied to the common electrode is 0 V, the voltage to be applied to the liquid crystal capacitance Clc1 becomes a gradation voltage VD (=VD−0 V). On the other hand, to the pixel electrode 123 that is electrically insulated, the voltage obtained by dividing the gradation voltage VD according to the capacity ratio between the liquid crystal capacitance Clc2 and the control capacitor Cc is applied. The voltage to be applied to the liquid crystal capacitance Clc2, namely, a voltage V1 to be applied between the common electrode and the pixel electrode 123 can be expressed as follows:V1=VD×{Cc/(Clc2+Cc)}  (1)
When applying the gradation voltage VD, while the gradation voltage VD is applied to the pixel electrode 121, the voltage V1 that is lower than the gradation voltage VD is applied to the pixel electrode 123. Therefore, the gradation voltage VD in which the liquid crystal located at the sub-pixel B starts to incline from the initial state is higher than the gradation voltage VD in which the liquid crystal located at the sub-pixel A starts to incline from the initial state. Thus, there is a difference in a threshold voltage (the voltage that the liquid crystal starts to incline from the initial state) between the pixel electrode 121 that is electrically connected to the source electrode (S) and the pixel electrode 123 that is insulated with the source electrode (S). As a result, the luminance gradation characteristic in the oblique direction of the liquid crystal display has been remarkably improved.
FIG. 8 is a graph showing a luminance characteristic (a gradation luminance characteristic) to an input gradation of the liquid crystal display shown in FIG. 7. In the drawing, a horizontal axis represents the input gradation (gray scale) and a vertical axis represents the luminance (T/Twhite), which is standardized at the luminance (Twhite) upon a white display. In the drawing, a curved line represented by a solid line represents a gradation luminance characteristic at a direction vertical to the display screen of the liquid crystal display shown in FIG. 7 (hereinafter, referred to as “a square direction”) and a curved line connecting black boxes represents a gradation luminance characteristic in a direction at an azimuth angle of 90° and a polar angle of 60° to the display screen (hereinafter referred to as “an oblique direction”). In the drawing, the curved line connecting black triangles represents a gradation luminance characteristic in the oblique direction of the liquid crystal display in the conventional vertical alignment system in which the pixel electrode is not divided as a comparative example. In this case, the azimuth angle is defined as an angle that is measured in a counterclockwise direction with reference to the direction to the right of the display screen. A polar angle is an angle to a line vertical to the center of the display screen.
As shown in FIG. 8, in the gradation luminance characteristic in the square direction, the more the input gradation is, the more the luminance is flatly increased and the curved line showing this characteristic is convex downward. On the contrary, in the gradation luminance characteristic in the oblique direction of the conventional liquid crystal display, the luminance in the oblique direction is higher than the luminance in the square direction in the range of gradation levels about 0 to 210, however, the luminance in the oblique direction is lower than the luminance in the square direction in the range gradation levels about 210 and over. On the curved line showing the gradation luminance characteristic in the oblique direction of the conventional liquid crystal display, a part that is largely convex upward and a part that is concave downward are mixed. As a result, when the display screen of the conventional liquid crystal display is seen from the oblique direction, the luminance difference between the input gradations is made small and this leads to generation of missing of gray scale or extending of gray scale and for example, the color of the image appears more whitish.
However, the luminance in the oblique direction of the liquid crystal display shown in FIG. 7 is higher than the luminance in the square direction across the all gradations. On the curved line showing the gradation luminance characteristic in the oblique direction of the liquid crystal display shown in FIG. 7, a part that is largely convex upward and a part that is concave downward are not mixed differently from the curved line showing the gradation luminance characteristic of the conventional liquid crystal display. Therefore, even when the display screen of the liquid crystal display shown in FIG. 7 is seen from the oblique direction, death and spread in the gradation are not generated and it is possible to prevent the color of the image from appearing more whitish. In this way, the gradation luminance characteristic in the oblique direction of the liquid crystal display shown in FIG. 7 has been remarkably improved as compared to that of the conventional liquid crystal display in which the pixel electrode is not divided. Further, in the liquid crystal display having the sub-pixels A and B in one pixel, it is preferable to make the ideal applied voltage of the sub-pixel B at the side of the high threshold voltage about 0.6 to 0.8 times of the applied voltage at the side of the low threshold voltage of the sub-pixel A.
[Patent document 1] JP-A-2004-134954
[Patent document 2] JP-A-2004-071178
[Patent document 3] JP-A-2004-265552
[Patent document 4] JP-A-2003-149647
The gradation luminance characteristic in the oblique direction is improved by the pixel structure of FIG. 7. However, since the voltage applied to the liquid crystal arranged at the sub-pixel B is decreased than the gradation voltage VD as shown in the expression (1), the luminance of the display screen has been decreased. In order to control this lowering of the luminance at the minimum, in the liquid crystal display shown in FIG. 7, the gradation voltage VD (a white voltage) for displaying white by the conventional liquid crystal display is set higher. However, this involves a problem shat a response speed of the liquid crystal is delayed when the white voltage is set higher.